1. Field of the Invention
The present invention relates to polymer blends, and more particularly to polymer blends comprising methyl methacrylate copolymer and aromatic polycarbonate.
The polymer blends obtained by the present invention have excellent transparency and used beneficially as transparent materials having characteristics of both polymethyl methacrylate and aromatic polycarbonate and as films, plates and like shaped articles, in particular as optical shaped articles such as lenses and optical disk substrates.
2. Description of the prior Art
Polymethyl methacrylate has excellent characteristics in transparency, weather resistance and surface hardness. This resin however has the problems of insufficient thermal resistance and mechanical properties when used for specific purposes, high water absorption and the like and improvement of these problems has been desired. On the other hand, aromatic polycarbonates, in particular one obtained from Bisphenol-A and phosgene (hereinafter referred to as "Bisphenol-A polycarbonate") have excellent mechanical properties, thermal resistance and transparency, but have the drawbacks of large residual stress and birefringence caused by poor melt flow during their processing. Blending the two resins has been attempted for the purpose of obtaining a transparent resin having both of their excellent characteristics. For example, a transparent, completely compatible blend is obtained by mixing polymethyl methacrylate and Bisphenol-A polycarbonate in a common solvent of tetrahydrofuran and then casting the solution into film. However the obtained film separates into two phases since the cloud point of the blend of the two resins is about 180.degree. C. at most. It is known that melt blending of polymethyl methacrylate and an aromatic polycarbonate therefore cannot provide a completely compatible blend but gives a blend having an opalescent luster, thus being not transparent (see J. Polym. Sci., Polym. Phys. Ed., 25, 1459 (1987)).
Attempts have also been made to prepare blends of aromatic polycarbonate with methyl methacrylate copolymers having introduced various comonomer units for the purpose of improving compatibility at high temperatures. For example, U.S. Pat. No. 4,749,749 discloses that a transparent blend is obtained by melt blending Bisphenol-A polycarbonate with a copolymer of methyl methacrylate and a methacrylamide, e.g. N-cyclohexylmethacrylamide. U.S. Pat. No. 4,950,716 discloses a transparent or only slightly cloudy blend obtained by melt blending Bisphenol-A polycarbonate with a copolymer of methyl methacrylate and a maleimide, e.g. N-cyclohexylmaleimide. However, since the methyl methacrylate copolymers described in U.S. Pat. Nos. 4,749,749 and 4,950,716 both contain nitrogen atoms, the above blends have, as described in Example of the latter, the drawbacks of ready discoloration upon melt molding and low cloud point which is unsatisfactory for melt blending at higher temperatures.
U.S. Pat. No. 4,906,696 proposes, to solve the above problems, to use as a comonomer for methyl methacrylate copolymer methacrylate and/or acrylate comprising cyclic hydrocarbon in the ester group, and if necessary also use styrene unit and/or .alpha.-methylstyrene unit. According to this U.S. Pat. No. 4,906,696, extrusion of a mixture of Bisphenol-A polycarbonate and a methyl methacrylate copolymer containing units from these comonomer gives a transparent blend, the cloud point of which is higher than that of a blend of polymethyl methacrylate and Bisphenol-A polycarbonate. The patent describes that for example a blend of Bisphenol-A polycarbonate and a copolymer of methyl methacrylate and phenyl methacrylate has a cloud point of 250.degree. C. or above.
Although there has been thus reported improvement of compatibility of a polymethyl methacrylate with an aromatic polycarbonate by modifying the polymethyl methacrylate by copolymerization, blends of known methyl methacrylate copolymers and aromatic polycarbonate cannot be said to have cloud points of sufficiently high level. That is, since these blends have, by the presence of polycarbonate being blended therein, higher melt viscosity than polymethyl methacrylate and their suitable processing temperatures hence shift to higher side, no known blends have sufficiently higher cloud point than their processing temperatures. The blend so far known to have the highest cloud point is the above-mentioned one of the aromatic polycarbonate and a methyl methacrylate copolymer containing units from phenyl methacrylate. Even this blend has a cloud point of about 250.degree. C. at most, which is insufficient for an appropriate, optional selection of processing conditions.
In blends as herein discussed, their physical property characteristics are, as well as the compatibility between the two constituting resins, of very great significance. The blend of Example 48 of U.S. Pat. No. 4,906,696, i.e. that using a copolymer prepared from methyl methacrylate and phenyl methacrylate, has, its glass transition temperature being as low as 117.degree. C., insufficient thermal resistance; while that of Example 63 using a copolymer prepared from methyl methacrylate, phenyl methacrylate and .alpha.-methyl styrene is, although its glass transition temperature is increased, of low cloud point. To summarize, while improvement in compatibility, particularly that at high temperatures, by the introduction of other comonomer units into polymethyl methacrylate has been reported, the improvement so far has been accompanied with the problems of discoloration upon blending, insufficiently high glass temperature and too large water absorption of the obtained blend, and the like. There has thus not been obtained a blend of polymethyl methacrylate and aromatic polycarbonate being excellent in all of transparency, thermal resistance and suppressed water absorption.